1. Technical Field
This application relates to computer storage devices, and more particularly to the field of transferring data between storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass. and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., U.S. Pat. No. 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units. The logical disk units may or may not correspond to the actual disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
In some instances, it may be desirable to copy or mirror data from one storage device to one or more others. For example, if a host writes data to a first storage device, it may be desirable to copy that data to a second storage device provided in a different location so that if a disaster occurs that renders the first storage device inoperable, the host (or another host) may resume operation using the data of the second storage device. Mirrored systems may be used for many purposes, ranging from preservation of real-time operations during back-up to data center migration, and mirrors may be located in any two or more different logical devices. An example of mirrored system capability is provided, for example, by the Symmetrix Remote Data Facility (RDF) product provided by EMC Corporation of Hopkinton, Mass. and further described, for example, in U.S. Pat. No. 5,742,792. With RDF, a first storage device, denoted the “primary storage device” (or “R1”) is coupled to the host. One or more other storage devices, called “secondary storage devices” (or “R2”) receive copies of the data that is written to the primary storage device by the host. The host interacts directly with the primary storage device, but any data changes made to the primary storage device are automatically provided to the one or more secondary storage devices using RDF. The primary and secondary storage devices may be connected by a data link, such as an ESCON link, a Fibre Channel link, and/or a Gigabit Ethernet link. The RDF functionality may be facilitated with an RDF adapter (RA) provided at each of the storage devices.
In any mirrored system, it is necessary to determine whether data will be written to all mirrors in a synchronous or near-synchronous mode, or whether a primary storage device will be written in advance of its mirrors (asysnchronous mode). There are advantages and disadvantages to both approaches. Thus, certain mirroring systems may allow or require switching between asynchronous and synchronous modes in certain circumstances. For example, RDF allows synchronous data transfer where, after data written from a host to a primary storage device is transferred from the primary storage device to a secondary storage device using RDF, receipt is acknowledged by the secondary storage device to the primary storage device which then provides a write acknowledge back to the host. Thus, in synchronous mode, the host does not receive a write acknowledge from the primary storage device until the RDF transfer to the secondary storage device has been completed and acknowledged by the secondary storage device.
A drawback to the synchronous RDF system is that the latency of each of the write operations is increased by waiting for the acknowledgement of the RDF transfer. This problem is worse when there is a long distance between the primary storage device and the secondary storage device; because of transmission delays, the time delay required for making the RDF transfer and then waiting for an acknowledgement back after the transfer is complete may be unacceptable.
It is also possible to use RDF in an a semi-synchronous mode, in which case the data is written from the host to the primary storage device which acknowledges the write immediately and then, at the same time, begins the process of transferring the data to the secondary storage device. Thus, for a single transfer of data, this scheme overcomes some of the disadvantages of using RDF in the synchronous mode. However, for data integrity purposes, the semi-synchronous transfer mode does not allow the primary storage device to transfer data to the secondary storage device until a previous transfer is acknowledged by the secondary storage device. Thus, the bottlenecks associated with using RDF in the synchronous mode are simply delayed by one iteration because transfer of a second amount of data cannot occur until transfer of previous data has been acknowledged by the secondary storage device.
Another possibility is to have the host write data to the primary storage device in an asynchronous mode and have the primary storage device copy data to the secondary storage device in the background (sometime called “ADP Copy mode”). The background copy involves cycling through each of the subdivisions and corresponding tracks of the primary storage device sequentially and, when it is determined that a particular block has been modified since the last time that block was copied, the block is transferred from the primary storage device to the secondary storage device. Although this mechanism may attenuate the latency problem associated with synchronous and semi-synchronous data transfer modes, a difficulty still exists because there can not be a guarantee of data consistency between the primary and secondary storage devices. If there are problems, such as a failure of the primary system, the secondary system may end up with out-of-order changes that make the data unusable.
Accordingly, in some instances, it is appropriate to switch between ADP Copy mode and synchronous or semi-synchronous RDF transfer modes to take advantage of the benefits of both systems. However, to complete the transition from ADP Copy mode to synchronous or semi-synchronous RDF transfer mode, it is necessary to have finished transferring all of the write pending data left over from the ADP copy mode. The overhead associated with this may be unacceptable in some situations. Thus, it is desirable to have an RDF system that transitions from ADP copy mode to synchronous or semi-synchronous RDF transfer mode without all of the overhead associated with transferring the left over write pending ADP copy data.